\usetikzlibrary{matrix, positioning}

\begin{tikzpicture}
  \matrix [matrix of nodes, nodes in empty cells,
  row sep={3em,between origins},
  nodes={text width=1em, align=center,
      minimum height=1em, anchor=base}] (m)
      {
          & \node (n1) [circle, draw, radius=1em] {$n_1$}; \\
          \node (n2) [circle, draw, radius=1em] {$n_2$}; & & \node (n3) [circle, draw, radius=1em] {$n_3$}; \\
          & \node (n4) [circle, draw, radius=1em] {$n_4$}; & & \node (n5) [circle, draw, radius=1em] {$n_5$}; \\
      };
  \node at ([xshift=1em]n5.north |- n1.east) {DAG};
  
  \draw [->] (n1) --(n2);
  \draw [->] (n1) --(n3);
  \draw [->] (n2) --(n4);
  \draw [->] (n3) --(n4);
  \draw [->] (n3) --(n5);
\end{tikzpicture}